It is standard practice in the manufacture of valve metal capacitors, particularly surface mount valve metal capacitors, to form a monolithic structure comprising an anode wire extending from an anode wherein a dielectric and charge collecting cathode is on the surface of the anode with the dielectric between the anode and cathode. The anode will typically have a roughened or increased surface area, on which the dielectric and cathodes are formed, so as to increase the capacitance of the device. The manufacturing process includes attaching an anode lead, extending from the anode, to a lead frame at a first location and attaching the cathode to a lead frame at a second location.
It is necessary for the anodic components and the cathodic components to be sufficiently separated to avoid electrical arcing as would be readily understood. This requirement creates a loss in volumetric efficiency since a significant volume of the ultimate capacitor does not contribute to capacitance. For example, with reference to FIG. 1A, the volume of the capacitor which surrounds the anode lead, 3, extending from the anode face from which the anode lead extends to the outer edge of the encapsulant, 8, provides no electrical purpose and only functions to provide a location for attachment of the lead frame, 4, to the anode lead with sufficient separation from the cathode layers to avoid damage during welding. This problem is exacerbated by the necessity to provide enough separation between the active area of the capacitive element and the weld, 9, in order to ensure that the effects of the weld operation, radiating unabated through the environment towards the sensitive and unprotected dielectric and cathodic layers, do not degrade the quality and performance of those layers. Shielding of the element from the weld process does not prove beneficial in reducing the occupied volume because practical limits of manufacturing precision prevent shortening of the distance required beyond that required without shielding. When multiple capacitive elements are combined into one capacitor the volumetric efficiency is even further eroded.
Electronic device manufacturers, who are the primary purchasers of surface mount capacitors, have a large installed manufacturing infrastructure tailored to mounting a surface mount capacitor onto a circuit board, or related element, to form an electrical sub assembly. Therefore, it is a necessity to provide capacitors which are structurally similar to surface mount capacitors as currently employed. Particularly the size, shape, and dimensions of the device must be consistent with the installed base for use in existing attachment locations. The electronics industry is also constantly seeking to miniaturize electronic devices, or extract greater capacity and capability from the same size devices. This forces the manufacturer of components, such as capacitors, to seek more functionality in a given volume. These contradictory requirements have led to the desire for a surface mount capacitor which has a higher volumetric efficiency, or capacitance per unit volume, while mimicking an industry standard surface mount capacitor in size and lead orientation. To address the loss in volumetric efficiency due to the anode attachment to its respective leadframe, some manufacturers have attempted to locate the attachment outside of the encapsulant. Some methods of connecting an anode extension to a preexisting external terminal, external to an encapsulant, have been proposed in U.S. Pat. Nos. 6,819,546 and 7,161,797 both of which are incorporated herein by reference. These methods involve forming a portion of the traditional lead frame material with the anode and cathode adhered to the leadframe, or equivalent, embedded in the encapsulant, and connecting the edge of the terminal to the exposed anode extension with a conductive layer applied onto the end of the device.
Other methods of construction are provided in U.S. Publ. Appl. No. 2010/0165547 which is incorporated herein by reference. Described therein is a device wherein the anode extension, and a portion of the applied conductive cathode, is exposed outside of the protective encapsulant. The end surfaces of the device from which the anode extension(s) and cathode layer are exposed are then flame sprayed, and subsequently made solderable, to create a terminal on each end of the device. This applied terminal material exists only on the end faces of the device, and does not have a significant presence on the bottom, or mounting surface, of the device. The terminals also cover the entire end faces of the device. This design represents a valve metal device with the terminal structure of a multi-layered ceramic capacitor (MLCC) device. These terminal configurations are undesirable in the art, as these devices are not interchangeable with the industry standard termination specifications for surface mount capacitors. Furthermore, these terminal configurations are undesirable because the terminals extend the full width of the device. Per industry standards, the mounting pad on the printed circuit board is always narrower than the device terminal as this provides a stabilizing effect on the device during the soldering process when mounting the device to a printed circuit board (PCB). When the terminal extends the full width of the device the mounting pad on the printed circuit board is wider than the device effectively requiring more space on the circuit board than can ever be filled by the capacitive device with this terminal configuration resulting in less than ideal volumetric efficiency. Thus, a device that has terminals that are significantly narrower than the width of the device requires mounting pads on the printed circuit board that are narrower than the capacitive device, and thus require less space on the PCB, resulting in greater volumetric efficiency of the PCB. It is preferred that a device would conform to the industry standard and preferably the device terminal would be 0.4 mm, or more, narrower than the device case. Terminal configurations in which the terminal reaches the top surface of the device, as those disclosed in U.S. Pat. Nos. 6,819,546 and 7,161,797, and U.S. Publ. Appl. No. 2010/0165547 are also undesirable due to a common condition of modern electronic devices exhibiting RF transmission, or those sensitive to external RF and EM interference, as in cellular telephones where conductive metal grounded shielding is placed over the circuit board to mitigate such problems. In these devices, the shielding can come into contact with the top of the devices mounted to the PCB. Therefore, devices with terminals reaching the top of the device provide a potential electrical path between the terminals and the grounded shield thereby rendering the device and the circuit inoperable.
Other methods of constructing surface mount solid electrolytic capacitors have been proposed such as those described in U.S. Pat. No. 6,185,091 which is incorporated herein by reference. These teachings still lead to volumetric inefficiencies. The focus is a construction with performance improvement related to its impact on an electrical circuit. The design requires the attachment of anode and cathode extensions. These teachings describe terminals that are mechanically attached prior to encapsulation. As described above this occupies space inside the encapsulation that lowers the volumetric efficiency of the device. In addition, no methods of attachment are taught and must be assumed to follow conventional methods of attachment that have no advantage in volumetric efficiency.
In addition to the volumetric efficiency advantage gained from the method of attachment of the anode and cathodes, the encapsulation method plays a large role in the devices final volumetric efficiency. Many ways are used in the art to produce a thin wall of encapsulant on the active element to protect the active element from the environment. Traditional methods include injection molding around the element such that the element is suspended within the injection molding cavity. This method fails in improving efficiency as features that are part of the process of suspending the elements in the cavities must be substantial enough to support the elements, and thus are incorporated within the device occupying space not used for active capacitance. An example of this space occupying material is the leadframe. The leadframe must extend inside the package to support the element and its ability to support the element is related to its thickness. Additionally, methods known as facedown use one side of the leadframe, opposite of the capacitive element, to be supported against the molding cavity, thus reducing its required thickness for supporting the element. However, decreasing the thickness of the facedown leadframe is limited by its ability to be adhesively bound to the encapsulant and by the thickness required to mechanically lock the leadframe with the encapsulant. If either of these aspects are lacking, due to trying to improve the volumetric efficiency, then the external forces on the capacitor terminals are transferred directly to the internal elements with potential of damage occurring. These factors limit the volumetric efficiency improvements, specifically around the thickness dimension of the device. Additionally, the leadframe configuration within the facedown design does not address the difficulty in controlling the encapsulant thickness on the side of the element opposite the facedown leadframe. This thickness is still controlled by the injection molding process.
The injection molding process is a process by which resin is brought into a cavity within which the capacitive elements have been suspended. This suspension is typically done by supporting portions of the traditional terminations of the leadframe or such, as in facedown packaging, by the compression of the leadframe down to a surface and the injection cavity located opposite of that surface. One issue with this process is that the resin is then presented to the cavity either directly within the capacitor cavity at the site of a long aspect ratio region or outboard of this capacitor cavity where resin does not enter at the site of a long aspect ratio region. If the resin is presented within a portion of the capacitor cavity the portion of the resin, which is in contact with the cavity portion of the device, must later be removed so as to not be part of the finished device. This removal process can both impact the electrical performance or the final target dimension capability and is made further difficult as the resin wall thickness is reduced in miniaturization. In addition, steps taken to remove this portion can add cost and complexity, especially when trying to control the final dimensions, as an ongoing goal in the art. If the injected resin is presented outbound of the cavity, then the major constraint to the process is the resin's ability to be flowed into long aspect ratio cavities. As is the goal in miniaturization, the resin wall between the capacitive element and external environment is preferably made as small as possible. In this case of standard capacitors in the industry it is possible to have regions of the cavity in which resin must flow 100 times or more in length versus the thickness of the wall attempting to be injected too. This long aspect ratio makes it difficult to fill due to back pressure created from the flow and its effects on the other flow characteristics in the remaining portion of the cavity, thereby risking more mixing of the resin adjacent to the heated cavity die possibly causing the material to cure prematurely.
It is possible to achieve process conditions, and materials, that allow for the long aspect ratio fills between the cavity and the elements, however, as stated this typically results in a high pressure being required to transfer the material across these long portions of the capacitive element and may add to the cost and complexity of the materials used. This high pressure can also create a series of unwanted characteristics. If the long aspect ratio exists on two sides of the capacitive elements, then the practicality of having evenly balanced pressures is poor and thus the capacitive elements may be biased to one side of the encapsulant. This bias takes away from the overall thickness potential of the final component as the final component will typically require a minimum encapsulant wall thickness. Further difficulties in processing parts with such a long aspect filling ratio is that the pressure created during the time the material is filling through the long aspect portion can result in stress being applied to the element risking damage to the element or forcing it to move within the package, resulting in poor quality and/or poor efficiency.
Another method of achieving good fill over longer aspect ratios is pre-filling the cavity with a liquid resin and lowering the cavity to a fixed height and forcing the liquid resin out of the cavity, leaving only the desired amount of resin defined by the lowering of the cavity and other fixed portions with the cavity. The limitation with this method is that when trying to achieve very thin walls of encapsulant the manufacturing variation of each capacitive element will limit the achievable wall thickness due to maintaining the minimum wall thickness for every part requires that the cavity thickness for the encapsulant be fixed for the largest of the variation within the capacitive elements. To process each element with varying height control is impractical and leaving some finished components with too thin of a encapsulant wall could have failures later in the process.
Other methods disclosed include the use of resin sheets to provide resin directly to the portions of the encapsulant so that the resin does not have to traverse the long aspect ratio portions of the device as described in U.S. Pat. No. 7,595,235 which is incorporated herein by reference. This process is helpful in reducing the effects of the encapsulant resin movement due to the resin being distributed over the capacitive element surface prior to flowing the resin into place. This method helps in reducing this stress at the encapsulation process but still lacks finer control of the encapsulant thickness without a method to control the final cavity dimensions that form the thickness of the capacitor.
As set forth above, there is an ongoing desire for a device with improved volumetric efficiency while maintaining the exact terminal configuration consistent with industry standards for valve metal surface mount capacitors. In spite of the extensive efforts there is still a desire for increased volumetric efficiency and improvements in the electrical performance of capacitors. Such improvements are provided herein.